The present invention relates to a thermal field emission electron gun and particularly to the electron gun having a thermal field emission cathode whose drift FE (field effect) noise is kept substantially small for a long period.
It has been known that the thermal field emission electron gun shows current fluctuations called FE chip noise, which is peculiar to a field emission cathode. The main purpose of prior-art apparatuses of this kind is observation of an image in SEM or the like generally, and therefore they are so constructed that the strength of an image signal changing with the current fluctuations can be corrected by subjecting an emission current fluctuation and the signal to electrical division. Besides, the image observation in SEM is executed in a cycle of several tens of minutes, and therefore these apparatuses have been employed in their own way without hindrance even when an emission current drift is large. Among generally-known examples relating to this electron gun, there is an example disclosed in Japanese Patent Laid-Open No. 62-93848 (1987), for instance.
Such a cathode employed in SEM or the like as stated above is of a CFE (cold field effect) type, and this cathode of the CFE type has the following problems.
(1) Ion collision due to an electric field
(2) Rise in temperature of a chip and fusion thereof due to emission of a current of high density
(3) Increase in work function due to chip contamination
Generally, ultra-high vacuum is maintained for coping with the above problems (1) and (3), and a proper amount of electric field is needed for coping with the above problem (2). However, ion collision energy is in the relation of E=qv (q: electric charge) in the CFE-type cathode, and therefore it increases as the charge q and a lead-out voltage v increase. This causes a large chip damage and an increase in a danger of discharge. For this reason, the electron gun using the CFE-type cathode is employed practically in the vacuum of 10.sup.-7 pa or less and with a lead-out voltage V.sub.l &lt;5 kV. Even for the CFEtype cathode having been put to practical use in this way, it is difficult to reduce the FE noise to 15% or below and the drift of the emission current to 10%/hr.
The present invention relates to the electron gun to be used mainly for an electron beam drawing apparatus, and therefore it is demanded that the FE noise itself is small and, besides, the drift thereof and of the emission current is low sufficiently. Accordingly the abovementioned CFE-type cathode is not fit for practical use therefor.
In this regard, a thermal field emission cathode (hereinafter abbreviated as TFE) is employed in the present invention. However, conventional TFE, as it is, has various problems in its application.
TFE using a Ti/W chip, which has been known heretofore, needs to be employed at a high temperature and has complicated structures of a grid and others, which causes difficulty in employment. TFE using a Ti/W chip of relatively low working temperature is unstable in the FE noise and it is difficult to keep the FE noise low stably for a long period.